如何避免在片段启动时每次触发loadIntent（）

Question

When using mosby-mvi,

Following code works fine -- fired only once when fragment created.

override fun loadIntent(): Observable<Unit> {
    return Observable.just(Unit)
}

Following code works unexpected -- fired each time the activity wake up from background.

override fun loadIntent(): Observable<Unit> {
    return Observable.just(Unit).mergeWith(RxView.clicks(error).map { Unit })
}

Why is that?

Answer 1

That is RxJava's contract that Mosby respects.

In the first example what happens is the following:

override fun loadIntent(): Observable<Unit> {
  return Observable.just(Unit)
}

Then in the presenter presenter you do something like:

intent(YourView::loadIntent)

This basically subscribes to loadIntent() which is Observable.just(Unit)

Then the following RxJava callbacks are triggered inside intent(YourView::loadIntent) :

--> onNext(Unit)
--> onComplete()

So your intent(YourView::loadIntent) reached the onComplete() state which means any future emissions will be just ignored (that is the RxJava contract). So the "magic" why this intent only triggers is because in your presenter it reaches onComplete() .

In the second example:

override fun loadIntent(): Observable<Unit> {
    return Observable.just(Unit).mergeWith(RxView.clicks(error).map { Unit })
}

The thing to notice here mergeWith() operator. The "problem" here is that mergeWith() only completes (and calls onComplete() ) if both Observable.just(Unit) and RxView.clicks(error) individually reach onComplete . While Observable.just(Unit) reaches onComplete() as already discussed in the previous code snipped, RxView.clicks(error) never reaches onComplete() and that is why mergeWith() never reaches onComplete() and that is why on Fragment recreation (like screen orientation change) this intent triggers again.

I would recommend to do the mergeWith() in your presenter and split it in two intents: loadIntnent() and retryLoadingOnErrorIntent() .